1. Field of the Invention
This invention relates to the separation of feed gas mixtures containing less readily and more readily adsorbable components. More particularly, it relates to such gas separation using pressure swing adsorption (PSA) processing.
2. Description of the Prior Art
In numerous chemical processing, refinery, metal production and other industrial applications, high purity gas streams are frequently employed for a variety of purposes. For example, high purity oxygen is used in various industries, such as chemical processing, steel mills, paper mills and in lead and glass production operations. Many such applications require purities in the range of about 90-93% oxygen at flow rates of up to 100,000 cubic feet per hour or more. While oxygen and nitrogen can be produced by various air separation techniques, PSA processing is particularly suited for air separation in a variety of applications, particularly in relatively small sized operations for which the use of a cryogenic air separation plant may not be economically feasible.
In pressure swing adsorption (PSA) processing, a feed gas mixture containing a more readily adsorbable component and a less readily adsorbable component is commonly passed to an adsorbent bed capable of selectively adsorbing the more readily adsorbable component at an upper adsorption pressure. The bed is thereafter depressurized to a lower desorption pressure for desorption of the more readily adsorbable component and its removal from the bed, prior to repressurization and the introduction of additional quantities of the feed gas mixture to the bed as cyclic adsorption-desorption-repressurization operations are continued in the bed. In vacuum pressure swing adsorption (VPSA) processing, the lower desorption pressure is a subatmospheric, or vacuum, desorption pressure. Such PSA/VPSA processing is commonly carried out in multi-bed systems, with each bed employing a PSA/VPSA processing sequence on a cyclic basis interrelated to the carrying out of such processing sequence in the other beds of the adsorption system. In PSA/VPSA systems designed for the recovery of high purity oxygen product as the less readily adsorbable component of air, each adsorbent bed will commonly contain an adsorbent material capable of selectively adsorbing nitrogen as the more readily adsorbable component, with said nitrogen being subsequently desorbed and removed from the bed upon reduction of the pressure of the bed from the upper adsorption pressure level to a lower desorption pressure.
Various processing approaches have been developed for applying the PSA/VPSA technology for practical commercial operations, such as the above-indicated production of oxygen from feed air. In one such approach, a two bed VPSA system, having adsorbent material capable of selectively adsorbing nitrogen from feed air, is employed in a processing sequence having six basic steps carried out in each bed, on a cyclic basis, interrelated to the carrying out of such steps in the other bed. Thus, each bed undergoes the following steps: (1) pressurization from lower subatmospheric desorption pressure to an intermediate pressure, with pressure equalization gas being passed from the top, or product end, of the other bed, which is undergoing depressurization, to the top of the said bed; (2) pressurization from intermediate pressure to upper adsorption pressure by the introduction of feed air to the bottom, or feed end, of the bed; (3) feed-adsorption-oxygen product recovery, in which additional quantities of feed air are introduced to the bottom of the bed, and oxygen, the less readily adsorbable component thereof, is withdrawn from the top of the bed for product recovery and for bed purge purposes; (4) cocurrent depressurization in which gas is released from the top of the bed and is passed to the top of the other bed, which is undergoing repressurization, for pressure equalization between the beds at an intermediate pressure range; (5) evacuation, or countercurrent depressurization, to a lower subatmospheric desorption pressure, with release of gas from the bottom end of the bed; and (6) purge at said lower desorption pressure. In order to synchronize the cyclic operation for the two beds of the system, steps 1-3 are carried out in one bed, undergoing depressurization from the upper adsorption pressure to the lower desorption pressure and regenerating, while steps 4-6 are carried out in the other bed, undergoing repressurization from said lower desorption pressure to the upper adsorption pressure and use for the production of product gas. The total cycle time is relatively short, typically about 60 seconds.
In the practice of this processing cycle, the pressure equalization of step (1) causes an increase in pressure of the bed being repressurized, typically, from a lower, subatmospheric, or vacuum, desorption pressure of about 0.35 atm to an intermediate pressure of about 0.67 atm, with this increase in pressure of about 0.3 atm utilizing about 5-20% of the total cycle time. During this pressure equalization step, it will be understood that the feed and vacuum blowers employed for the two bed VPSA system are unloaded.
During the bed repressurization of step (2), feed air is passed to the bed to cause the pressure thereof to increase from the lower intermediate pressure level reached during the initial pressure equalization to an upper adsorption pressure in the superatmospheric range, typically about 1.3 to 1.5 atm.
Upon reaching the desired upper adsorption pressure, additional quantities of feed air are passed to bottom of the bed at said upper adsorption pressure, in step (3), with the more readily adsorbable nitrogen component thereof being selectively adsorbed, and the less readily adsorbable oxygen product being selectively passed through the bed and withdrawn from the top of the bed. The oxygen thus withdrawn is recovered as the desired oxygen product, except for a portion thereof that is diverted for use as purge gas, generally in the other bed. The oxygen withdrawn initially is typically recovered as product gas, with a portion of the oxygen being diverted for purge purposes during the latter portion of said step (3). The bed repressurization of step (2), and the initial portion of step (3) in which the oxygen withdrawn is generally recovered as product gas, utilizes a portion of the overall cycle time that varies depending on the overall conditions of the embodiment, but that is usually in the range of about 30-40% of the overall cycle time. The further portion of step (3), in which additional oxygen product is recovered, and a portion of the oxygen is diverted for purge purposes, commonly requires about 15-30% of the overall cycle time.
During the countercurrent depressurization-pressure equalization of step (4), the pressure decreases to an intermediate pressure determined in accordance with the pressure in the other bed, which is undergoing repressurization as a result of the pressure equalization between the beds. As with step (1), the change in pressure from the start to the end of step (4) is approximately 0.3 atm, with the bed typically reaching an intermediate pressure level in the range of about 0.9-1.1 atm. In this regard, it should be noted that the pressure equalization between the beds is commonly terminated at a partial pressure equalization, rather than being continued to a full pressure equalization between the beds. Thus, in common practice, the beds do not completely equalize at the same pressure, with the difference in pressure between the beds upon completion of the partial pressure equalization typically being from about 1 to about 10, desirably about 3 to about 5, psi.
Following cocurrent depressurization-pressure equalization, the bed being depressurized is evacuated to a lower, subatmospheric desorption pressure in step (5) of the processing cycle. In this portion of the regeneration of the bed, it is typically evacuated, using a vacuum blower, to the desired lower desorption pressure, e.g. 0.35 atm, with release of gas, i.e. previously adsorbed nitrogen, from the feed, or bottom, end of the bed.
In step (6) of the processing cycle, purge gas is introduced to the upper, i.e., product, end of the bed at the lower desorption pressure, e.g. 0.35 atm to facilitate removal of nitrogen from the bottom of the bed, with the purge gas being the portion of the oxygen product gas from the other bed that is diverted for such purge purposes.
Following the regeneration of each bed in steps (3), (4) and (5), it is repressurized, in steps (1) and (2), and product oxygen is produced in step (3) of the cycle with additional quantities of feed air being introduced to that bed as the processing sequence is carried out, on a cyclic basis, in each bed.
Those skilled in the art will appreciate that there are many other PSA and VPSA processes used for the production of oxygen from feed air. Each such approach may include its own unique steps or features, or combinations thereof. The particular process described above provides for the economical recovery of high purity oxygen in the range of 90-95%. While the process described has thus provided desirable benefits, there is a continuing desire in the art for further improvements in the PSA/VPSA processing. Such improvements are desired to satisfy the ever-increasing need for higher performance levels in the economical supply of oxygen and other industrial gases for a wide variety of commercial applications. Specifically, further improvement is desired to expand the capacity of PSA/VPSA systems, to increase the efficiency of the PSA/VPSA process, and to reduce the unit power requirements of PSA/VPSA operations.
It is an object of the invention to provide an improved PSA, including VPSA, process for the separation of gases.
It is another object to provide an improved PSA/VPSA, process for the recovery of high purity oxygen by air separation.
It is a further object of the invention to provide a PSA, including a VPSA, process having enhanced efficiency and reduced unit power, and being capable of expanding the adsorptive capacity of the systems employed therein.
With these and other objects in mind, the invention is hereinafter described in detail, the novel features thereof being particularly pointed out in the appended claims.